The invention relates to an exhaust-gas turbocharger for an internal combustion engine having a compressor in the induction tract and an exhaust-gas turbine in the exhaust-gas line, and, in particular, to an exhaust-gas turbocharger which includes a plurality of spiral ducts which direct exhaust gas to the turbine.
Such an exhaust-gas turbocharger is described in publication DE 42 42 494 C1. The turbocharger comprises an exhaust-gas turbine, which in a turbine housing, has a turbine wheel which is driven by the exhaust gases of the internal combustion engine. The exhaust gas is fed to the turbine wheel via a plurality of spiral ducts which direct the exhaust gas to the turbine wheel via in each case a radial flow inlet passage. In DE 42 42 494 C1, exemplary embodiments with two, three and four spiral ducts are described, to which in each case radial flow inlet passages are assigned in a distributed manner in various angular sectors over the circumference of the turbine wheel. Located in each flow inlet passage is a guide cascade which provides for a defined incident flow to the turbine wheel with adjustable swirl and volumetric flow. On account of the guide cascades, provided in various ways, in each flow inlet passage, the different damming behavior in the individual spiral ducts can be used for different applications. For example, in order to assist exhaust-gas recirculation, a guide cascade having a narrow passage of flow may be used in a first spiral duct for producing a high dynamic pressure. On the other hand, a guide cascade having a larger cross section of flow may be provided in a second spiral duct in order to make possible a largely choke-free exhaust-gas inflow to the turbine wheel, in particular during part-load operation of the internal combustion engine.
The different guide cascades are arranged on a common sleeve-shaped flow guide member. The flow guide member is designed to be axially displaceable, as a result of which wall sections of the flow diffuser which are of closed design and are continuous or free of interruptions can be pushed into individual flow inlet passages, and the effective flow cross section can accordingly be reduced. In this case, however, it has to be noted that the flow conditions can deteriorate significantly if a guide cascade is only partly exposed and if a flow inlet passage is partly closed by the wall of the flow guide member, as a result of which, in particular, the efficiency of the exhaust-gas turbocharger is impaired. For this reason, the flow guide member is normally either shifted into a position in which the flow inlet passage is completely closed or into a position in which the guide cascade extends over the entire axial width of the flow inlet passage.
It is an object of the invention to provide additional adjusting possibilities for an exhaust-gas turbocharger of the generic type using simple design measures. In particular, without impairing the flow conditions, it is to be possible to set intermediate positions in an infinitely variable manner for the axial width of the inlet flow passage to the turbine wheel.